This proposal is directed towards the parallel development of new fluorescent probes and imaging methodologies to permit quantitative single molecule sensitivity in living cell systems. The long-term goal is to improve the overall detection sensitivity, and labeling specificity to allow imaging of single molecules within living tissues and whole animals. New fluorescent probes in the visible and infrared spectral regions will be developed based on our established expertise in three areas, genetically-encoded proteins, lanthanide chelates, and nanocrystals (also called quantum dots). Each of these approaches will be tested in an "apples-to-apples" comparison for imaging of a plasma membrane surface target (using the angiotensin receptor as our model system) and an intracellular target (using the mitochondrial peripheral-type benzodiazepine receptor (PBR) as the model system). The majority of our proposed effort will focus on sub-cellular resolution fluorescence imaging by widefield, deconvolution, confocal, and multi-photon excitation microscopy, for which we have all the required instrumentation already in place. To further enhance the sensitivity of detection, we will implement new detection strategies based on spectral and time-gated resolution. To reach extremely high-resolution, we will also determine the utility and limitations of using each of these probes for direct detection by electron microscopy for correlative imaging. Finally, looking towards future in vivo imaging applications, we will develop new red and near infra-red (NIR) imaging approaches that will be made possible with our new probes. This research will provide tools for describing cell physiology in terms of specific molecular dynamics and interactions, which will lead us into the next frontier of structural cell biology.